This invention finds use in the field of medicine and relates to a method and apparatus for applying periodic pulsation to a liquid flowing through a liquid-feed conduit.
A low-temperature perfusion preservation method is known as a method of preserving a surgically removed living organ such as the liver, heart, pancreas and kidney until the organ is implanted. This is a method of prolonging the survival period of the living organ by controlling the tissue metabolism of the organ at a low temperature of 3.degree.-10.degree. C., perfusing an oxygenated perfusate through the artery and/or portal vein of the organ while the amount of oxygen consumed is kept low, replenishing the organ with oxygen and nutrients that are particularly required by the organ, and removing metabolic waste matter.
In this method of preserving said organ having a vascular system, perfusing the oxygenated perfusate sufficiently up to the periphery of the vascular system is particularly important in terms of maintaining the function of the organ. In order to achieve this peripheral perfusion, a pulsatile flow is more effective than a steady flow as a method of supplying the perfusate since the pulsatile flow makes it possible to obtain a high pressure sufficient for opening the capillary vessels of the organ constricted by low temperature.
Furthermore, even in cases where blood is perfused through the artery of a living body, a pulsatile flow approximating the normal blood flow waveform (the condition of the living body) is better in terms of the effects on the living body than a steady flow as a method of supplying the blood.
In order to attain these ends, a number of apparatus have been proposed in the prior art for the purpose of providing pulsation to a liquid flowing through tubing. For example, U.S. Pat. No. 4,598,697 discloses a pump apparatus for feeding blood, in which the blood is forced to pulsate by intermittently rotating rollers of a roller-type pump, by a prescribed angle, and U.S. Pat. Nos. 3,639,084 and 4,250,872 disclose an apparatus in which a pulsatile flow is produced in tubing by pressing a flexible tube, which constitutes part of a conduit, by a high-pressure fluid such as air or oxygen.
Pulsation internally of the artery of a living body can be classified into several forms depending upon the waveform of the pulsation. For example, as shown in FIG. 5 of the accompanying drawings (in each of the drawings, the abscissa corresponds to the time and the ordinate corresponds to the blood flow rate measured by utilizing the Doppler effect or an electromagnetic flow meter), blood flow waveforms of a peripheral artery affected by arterial obliteration can be classified broadly into five types. FIG. 5(A) is referred to as a 0-type waveform in which a back-flow waveform exists. This is a normal waveform in the thigh artery. With the progress of arterial obliteration, the waveform of blood flow gradually changes in the manner of FIGS. 5(B), (C), (D) and (E), where the waveforms are referred to as types I, II, III and IV, respectively. The type-I waveform is characterized in that the back-flow waveform has vanished from the normal waveform, and the waveforms from the type-II waveform onward are characterized by gradual decreasing and flattening of the wave crest and a gentler slope of the descending leg.
The present invention is based on the belief that the waveform of perfusion is related to the functional maintenance of microcirculation in the hepatic artery under low temperatures. The inventors have conducted vigorous research into the relationship between preservation of an extracted liver and the waveform of perfusion based on the classification of blood-flow waveform, shown in FIG. 5, which characterizes the arterial obliteration seen at peripheral arteries such as the thigh artery. As a result of this research, it has been found that though the normal waveform of blood supplied to the hepatic artery in a living body approximates the type-II waveform of FIG. 5, perfusion of the hepatic artery by a pulsatile waveform of types 0 through II, and preferably of types 0 and I, provides for excellent preservation effect on the liver when the low-temperature preservation of the extracted liver is employed.
In the conventional apparatus mentioned above, however, a waveform obtained thereby is not widely controllable for lack of controlling waveform distortion, described below, and it is impossible to obtain pulsation of the 0 type in such a manner that the aforementioned back-flow wave will appear.